1. Field of the Invention
This invention relates to a high heat-resistant catalyst support used for a catalyst for purifying automotive exhaust gases, and having excellent heat resistance and excellent sulfur-poisoning resistance.
2. Description of Related Arts
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NOx) to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat-resistant supporting base material formed of cordierite, a porous catalyst support layer formed of gamma-alumina and disposed on the supporting base material, and a noble metal catalyst ingredient such as platinum (Pt) and rhodium (Rh) loaded on the porous catalyst support layer. Further, the 3-way catalysts are also known in which ceria i.e., cerium oxide having an oxygen storage ability is employed in addition to the above catalyst ingredient in order to improve catalytic activity at low temperatures.
In the meanwhile, carbon dioxide (CO.sub.2) in exhaust gases from internal combustion engines of automobiles and the like has become a problem in view of global environmental conservation. Lean burn in oxygen excessive atmosphere is desired as a means of dissolving the problem. The lean burn improves fuel consumption, and as a result the amount of fuel used is decreased and CO.sub.2, which is included in combustion exhaust gas, can be suppressed from generating.
In this respect, the conventional 3-way catalysts aim to oxidize CO and HC and reduce NOx simultaneously into innocuous entities when the air-fuel ratio is at the stoichiometric point (or the ideal air-fuel ratio), and cannot exhibit sufficient reduction and removal of NOx in the exhaust gases which contain oxygen in excessive amounts required for oxidizing carbon monoxide and hydrocarbons therein at the time of lean burn. Hence, it has been desired to develop catalysts and exhaust gas purifying systems which are capable of adequately purifying NOx even in oxygen excessive atmospheres.
In view of the aforementioned circumstances, the applicants et al of the present invention have proposed an exhaust gas purifying catalyst in which alkaline-earth metals and platinum (Pt) are loaded on a porous support formed of alumina and the like in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, an exhaust gas purifying catalyst in which lanthanum (La) and platinum (Pt) are loaded on a porous support in Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860, and an exhaust gas purifying catalyst in which alkali metals and platinum (Pt) are loaded on an alumina support in Japanese Unexamined Patent Publication (KOKAI) No. 6-31,139. In using these catalysts, NOx are adsorbed on oxides of alkaline-earth metals or lanthanum oxide on the fuel-lean side (i.e., in the oxygen excessive atmospheres), and the adsorbed NOx react with reducing components such as HC and CO at the stoichiometric point or on the fuel-rich side (i.e., in the oxygen-lean atmospheres). So, these catalysts attain excellent NOx purifying performance even on the fuel-lean side.
By the way, the average temperature and the maximum temperature of inlet gases to the bed of catalysts tend to rise more and more in recent years due to severe restrictions on exhaust gases and the improvement in the capacity of engines. Therefore, it is desired to make a further improvement in the heat resistance of catalysts for purifying exhaust gases. Further, with an increase in inlet gas temperatures, it is also desired to improve NOx conversion at elevated temperatures.
In the conventional catalyst, however, there arises a problem that NOx storage components react with catalyst supports at high temperatures and as a result the NOx storage ability of the NOx storage components is deteriorated. Besides, in the conventional catalyst, a temperature range in which the maximum catalyst performance is obtained, namely, a temperature window is narrow, and the NOx conversion is hardly secured at elevated temperatures.
Further, in the conventional catalyst, the NOx storage components are poisoned by SOx which are produced from a very small amount of sulfur contained in fuel, in other words, the NOx storage ability is decreased due to sulfate generation. Consequently, the catalyst are degraded in durability.
Moreover, in the conventional catalyst, the NOx storage components have a poor dispersion. As a result, crystallization of the NOx storage components is promoted at and around a part having a high concentration of the NOx storage components, so that the NOx storage ability is deteriorated. The NOx storage ability, especially at elevated temperatures is greatly influenced by the combination of NOx storage components and catalyst supports, and the dispersibility of NOx storage components. Further, when the NOx storage components are poorly dispersed, sulfate crystals generated by sulfur poisoning easily grow and therefore, become more difficult to be removed, so that the durability of the catalysts is decreased.
It is supposed that the catalyst support comprises composite oxide constituted by NOx storage components and alumina in order that NOx storage components are highly dispersed. In the catalyst using such catalyst support, NOx storage ability is improved at high temperatures. However, there are disadvantages that NOx storage components are gradually scattered by long high-temperature endurance test, and that durability cannot be obtained satisfactorily. Further, when too much amounts of sulfur is contained, the catalyst is degraded by sulfur poisoning, and the performance of the catalyst is deteriorated.